1. Field
The invention relates to a method for determining the gas volume flow in gas recirculation at a filling station and to a device for carrying out this method.
2. Discussion of Prior Art
When carburettor fuels are transferred from tank to tank at a filling station, hydrocarbons evaporate, specifically, for example, in a mass which corresponds approximately to 1 per thousand of the liquid mass. Previously, these hydrocarbon vapours were discharged as loss into the atmosphere. Since the benzenes contained in the hydrocarbon gas are harmful to health and the hydrocarbons act as greenhouse gases, emissions of this kind have to be reduced. This was stipulated in principle for all European countries in the Directive 94/63/EC. In most European countries, this directive was implemented in national regulations. There are also comparable regulations in other countries, for example in the USA.
The reduction of emissions is brought about by means of a recirculation of the hydrocarbon gases during the respective tank transfer operation. When a tanker vehicle is filled at the refinery and when a storage tank (underground tank) is filled at a filling station, an additional gas recirculation hose (gas displacement line) is connected. This is designated by “Stage I gas recirculation”.
Gas recirculation during the filling of a motor vehicle at a filling station is called “Stage II gas recirculation”. Since in this case the petrol pump valve does not come to bear, gas-tight, against the filler neck of the motor vehicle to be filled, the hydrocarbon gases are sucked in by means of a pump at the inlet of the petrol pump valve (active gas recirculation). The volume of the recirculated gas mixture at the suction-intake point should be equal to the liquid volume introduced into the tank of the motor vehicle, so that hydrocarbon gases escape into the atmosphere as little as possible. There should also not be too much which is sucked away, since a greater gas volume would otherwise be recirculated into the storage tank than liquid volume which has been extracted, which would lead to a pressure rise in the storage tank. Emissions would consequently arise at the ventilation mast of the storage tank, which likewise has to be avoided.
Active gas recirculation is implemented predominantly as an autonomous system, that is to say independently of a monitoring device which checks the gas recirculation rate. The gas recirculation control electronics for a petrol pump are connected to an output of the petrol-pump computer which delivers a pulse rate which is proportional to the instantaneous flow of the fuel, that is to say to the rate of conveyance of the liquid fuel which, during a refuelling operation is pumped into the tank of the motor vehicle to be refuelled, this being a liquid volume flow. By means of a balancing curve based on calibration data, the gas recirculation control electronics either calculate a control signal for a proportional valve or else generate a control signal which influences the rotational speed of the electric motor of a gas-conveying pump. In both instances, the gas throughflow (that is to say the gas volume flow) is set as a function of the fuel throughflow (that is to say, of the fuel volume flow), such that the recirculation rate (defined as the gas throughflow/fuel throughflow) is approximately 100%.
The balancing of gas recirculation, that is to say the determination of the calibration data mentioned, takes place, as a rule, by means of an auxiliary set-up. In this case, a volume-flow meter (for example, a positive-displacement gas meter) is connected, gas-tight, to the suction intake port of a petrol pump valve. A control signal for the rate of conveyance of gas recirculation is then generated and the associated gas throughflow is measured. This is carried out in a sequence for the overall throughflow range. The calibration data thus obtained (assignment of the control signals to the respective gas throughflow) are stored in a table in a non-volatile memory in the gas recirculation control electronics. After the removal of the auxiliary set-up, the gas recirculation system works, during normal tank-filling operation, with the balancing table generated according to this method.
Gas recirculation systems of the type described are monitored for their correct functioning by means of automatic monitoring systems. In this case, a gas throughflow sensor inserted into the gas recirculation line is connected to evaluation electronics of the gas recirculation monitoring. Furthermore, the said fuel volume flow pulses serving for activating the gas recirculation control electronics are applied to the evaluation electronics of the gas recirculation monitoring. This determines the recirculation rate from these and from the measured gas throughflow.
A gas recirculation system is known from U.S. Pat. No. 6,170,539 B1, in which the gas flow is measured and used for regulating the gas recirculation.
For an exact monitoring of the gas recirculation, it is necessary to know the gas volume flow in the region of the petrol pump valve, since, when a motor vehicle is being refuelled, exactly as much gas volume is to be sucked away from the tank of the vehicle per unit time as fuel volume is introduced. The gas sucked away is routed inside the petrol pump hose provided with a coaxial line and is then transported further within the petrol pump in a specific line. The gas-conveying pump is located there, which ensures a pressure difference in order to cause the gas throughflow. The gas then flows further on to the storage tanks of the filling station. The gas throughflow sensor may be installed, for example, upstream of the said proportional valve, but also, for example, downstream of the gas-conveying pump (this is particularly when only one gas throughflow sensor is provided in a petrol pump having a plurality of filling outlets).
Various heat exchange processes take place in the line routing described. For example, the fuel flows at the temperature of the storage tank (underground tank) of approximately 15° C. to the petrol pump valve, so that the gas which flows back inside the petrol pump hose and essentially has the temperature of the motor vehicle tank is heated in winter and cooled in summer. The gas volume flow changes as a result. Furthermore, considerable dynamic pressure drops occur in the lines and likewise influence the gas volume flow. In this case, there may even be adiabatic processes, this, in turn, influencing the gas temperature. All of this has the effect that the gas volume flow at the location of the gas throughflow sensor does not coincide with the gas volume flow actually to be determined in the region of the petrol pump valves. The influence of these diverse effects on the gas volume flow cannot be detected particularly accurately and therefore a measurement of the gas volume flow inside the petrol pump leads to considerable uncertainties.